Methamphetamine (MA) misuse has increased dramatically in the United States during the past decade. Nevertheless, efficacious pharmacotherapies for MA dependence remain elusive despite extensive research on the neurobiology of the effects of amphetamines. Naltrexone (NTX) is an opioid receptor antagonist with empirically supported efficacy and FDA-approval for the treatment of alcoholism. A recent placebo-controlled study has suggested that NTX may be promising for the treatment of amphetamine dependence as it significantly increased abstinence, measured by amphetamine-negative urine samples, compared to placebo. This New Investigator R21 seeks to (a) examine the biobehavioral mechanisms of action of NTX for MA use disorders; and (b) test the pharmacogenetics of NTX for these disorders. We propose to recruit 50 non- treatment seeking individuals who meet criteria for MA dependence. Participant will complete two double- blinded, within-subjects MA administration laboratory sessions, one after taking NTX (50 mg/day) and one after taking placebo for four days. It is hypothesized that NTX will blunt MA-induced reward and craving and will improve response inhibition. In addition, we hypothesize that genetic polymorphisms of the mu, kappa, and delta opioid receptors will be useful in identifying responders to NTX and individual differences in MA-induced reward. The successful completion of this application will provide an initial characterization of the mechanisms of action of NTX among MA abusers and its pharmacogenetics. The long-term objective of this research is to develop and optimize pharmacotherapies for MA dependence by combining behavioral pharmacology and pharmacogenetics to elucidate the mechanisms of action of NTX for MA use disorders and their genetic bases.